Drill bits employing rotary cutters have been used for drilling for many years. The rotary cutters rotate about pins on the drill bit body and have an outside surface that includes cutting structure. This structure commonly takes the form of tungsten carbide inserts. The rotation of the cutter in the cutting area pulverizes the rock of other material for ready removal from the hole by circulating drilling mud.
The cutters are typically secured for rotation to the bit body through a bearing assembly. A bushing is also provided between the bit body and the cutter for support. In the past, the bearing assembly has been sealed from the drilling environment by means of a Belleville seal assembly or an O-ring.
Pressure compensating devices have also been developed to maintain the pressure differential across the Belleville or O-ring seal at a relatively low level to reduce contamination of the bearing under the high pressures encountered in drilling.
While O-ring type seal assemblies have improved the operation of the typical rotating cutter drill bit, the seal assembly has an active service life typically within the range of 100 to 200 hours of drilling. The seal assembly will degrade and permit drilling mud and other fluids present about the drill bit to enter the bearing assembly. Once the bearing assembly is contaminated, the service life is very short. The drilling mud and other fluids about the drill bit commonly contain impurities which quickly abrade the surfaces in the bearing assembly.
The drill bit may also pass through a layer containing hydrogen sulfide. The hydrogen sulfide typically is entrained in the drilling mud in bubble form and quickly acts to destroy the integrity of the O-ring seal assembly. It is common to replace the drill bit whenever a hydrogen sulfide pocket is encountered because the integrity of the seals is almost invariably destroyed.
In an effort to provide longer service life for a drill bit, an effective combination of materials has been developed. The pin or extension of the drill bit body forming the race for the bearings supporting the rotary cutter is formed of forged steel with the bearing race carburized. The bushing between the pin and cutter is formed of a copper alloy. A cobalt alloy surface layer is provided on the pin for contacting the copper alloy bushing. The sealing surfaces of the roller cutter and body against which the O-ring seal acts are formed of a smooth surface having approximately a ten micro inch finish.
Thus, a flat annular surface is formed on the inner surface of the extension of the drill bit body surrounding the slender bolt bearing pin. This annular surface is located in a plane parallel with the plane of the back face of the cutter.
Running in drilling mud, this flat annular surface is outside the sealing point and is subject to a corrosive environment which has been found to initiate and propagate cracks as a result of high stress in the area on the upper side of the bearing pin at the juncture with the drill bit body. It has been well established that in a cyclic loading situation such as found with the running of a drill bit, where a high cyclic tensile stress is produced, fatigue cracks are initiated and propagated at a greatly accelerated rate if a corrosive environment is present.
The cost of replacing a drill bit is expensive and the operation time consuming. Even worse, should a drill bit fail within the hole, resulting in cutter parts or other debris remaining in the hole, a very expensive and time consuming fishing operation is required. Therefore, a need has been shown to improve the service life of the rotary cutter type drill bit while retaining the advantages of its present design and material construction.